Epoxide-Diamine Crosslinked Polymer Networks as Sodium Metal Battery Electrolytes

Electrode volume changes often underpin short-circuits in rechargeable alkali batteries leading to their failure. In the ongoing quest for solid-state sodium batteries, polymer electrolytes (PEs) have received significant attention with the major focus being on polymer modulus alone. However, the recurring electrode volume changes necessitate polymer toughness to be given equal consideration in PE design, which has thus far received little attention. To this end, we report a comb-chain-type polymer network system comprising crosslinked poly(glycidyl methacrylate) (PGMA) and diamine-terminated-poly(ethylene glycol) (PEG) chains doped with sodium bis (fluorosulfonyl imide) (NaFSI) salt. We study the interrelationship between polymer network structure, ion conductivity, and mechanical properties of this chemistry by exploring two PGMA:PEG molar ratios (1:1 and 5:1), and three salt concentrations (Na+ to EO: (r) = 0.01, 0.0625, and 0.25). We observe that lower PGMA content results in higher hydrophilicity and lower crosslink density. These properties translate to a total ion conductivity of 0.5 × 10–4 S/cm at 25 °C and nearly 0.5 × 10–3 S/cm at 65 °C for the optimum composition (1:1) and salt concentration (r = 0.0625). The polymer electrolytes exhibit good mechanical properties (elongation at strain between 125 and 140%, Young's modulus between 0.3 and 0.7 MPa, and toughness between 0.25 and 0.70 MJ/m3). By virtue of their high ion conductivity and toughness, these comb–chain network SPEs have the potential to exhibit excellent cycling performance in emerging sodium metal batteries.